1. Field of the Invention
The present invention relates to an improved apparatus and method of treating surface or ground water i.e., "raw water" to water that is safe for human consumption, i.e., "potable water." The invention also relates to an improved apparatus and method of tertiary treatment of waste water so that it can be discharged in compliance with environmental laws and regulations. Waste water and raw water shall be referred throughout this document as "untreated water." More specifically, the invention provides for measuring the two parameters of color and turbidity of the untreated water entering the treatment plant, using a mathematical formulation to determine the dosage of coagulant suitable for the removal of impurities in the untreated water based upon the measured parameters, and a control action, i.e., addition of coagulant.
2. The Prior Art
It is known that the addition of coagulating agents such as alum cause the impurities such as suspended solids and dissolved organic wastes, to coagulate together to form larger particles. If an adequate quantity of coagulant is added, the process continues through flocculation, i.e., the formation of larger particles, eventually resulting in the particles to settled out or precipitate out of the water during treatment. The precipitation step is also called sedimentation. The addition of excessive amounts of coagulants can disrupt the coagulation process by lowering the pH of the untreated water, cause filters to become clogged and incur excessive treatment costs.
There have been several prior art methods of controlling the dosage of coagulants added to untreated water for the removal of impurities. The most common and accepted methods determining the proper coagulant dosage are the Jar Test, the monitoring of the Zeta Potential of water after treatment and the Streaming Current method of measuring particle charge.
The Jar Test method simulates in the laboratory the treatment plant processes of coagulation, flocculation and sedimentation in the laboratory. Effective coagulant addition is determined by such parameters as turbidity reduction, color removal and particle counts. The Jar Test is universally recognized as the most commonly used method for coagulation control. The Jar Test can be a very reliable procedure but is time consuming and is not capable of automatic dosage control. The test requires approximately 20 to 30 minutes to be conducted. During this time period, the temperature of the sample water is likely to change and the differing water temperature can impact the determination of the correct quantity of coagulant that should be added to the untreated water. The reliability of the jar test method can also be dependent upon the judgment and diligence of the operator conducting the test.
The Jar Test method requires samples of the untreated water entering the treatment plant to be placed in multiple sample jars (typically 6) and varying measured amounts of coagulants added to each sample jar. An appropriate period of time for mixing and settling is allowed to elapse and the jars are compared for relative clarity of the sample water and quantity of precipitate. An accepted procedure for this test method is described in the American Water Works Association publication entitled Water Quality and Treatment, A Handbook of Community Water Supplies, 4th Edition.
The Zeta Potential and Streaming Current methods are the two major instrumental methodologies utilized to determine the optimum dosage of coagulating agents to be added to the untreated water. Both methods make indirect measurement of the charge on the particles. In contrast, the invention subject of this application is not dependent upon any measure of charge.
The Zeta Potential of particles in untreated water is typically form -20 to -40 mV. If a suspension is destabilized by a charge neutralization mechanism, the Zeta Potential will be close to zero. Zeta Potential is normally conducted on coagulated particles after the rapid mix or at the low mix cycle during the jar test. The optimum dosage is determined when the Zeta Potential of the particles is close to zero. It should be noted that the Zeta Potential measures the charge characteristics of particles, but will not tell if sufficient concentration of coagulated particles, i.e. floc, is available for flocculation. An additional major limitation of this method is that it is performed only at the completion of the treatment of the water and therefore does not provide timely information for the treatment of the tested water. There is typically a 3 to 4 hour time lag between water entering the plant and the completion of treatement. This method is further limited by the necessary assumption that the untreated water entering plant exhibits the same characteristics as the tested water, the tested water having entered the plant typically 3 to 4 hours earlier.
Streaming Current is also a method of measuring particle charge. This method uses a cylinder with a piston that is reciprocating where charged particles are drawn in the cylinder. An alternating current is developed by the piston moving up and down and is called a Streaming Current. This alternating current is directly proportional to the charge on the particles and correlates with the Zeta Potential. The Streaming Current is used as a continuous monitoring device and may be designed to adjust coagulant dosage. However, the Streaming Current method is effective only when the pH of the tested and untreated water remains constant.
An alternate proprietary treatment method has been referenced in an article entitled Automating Control of Coagulant Dosing and appearing in the February 1998 World Water & Environmental Engineering. Although the specific provisions of this method claimed to have been utilized in the United Kingdom are not disclosed, this method can be distinguished from the present invention in that the proprietary method utilizes data "feed back" from the treated water, such as the apparent color, to adjust the treatment of the untreated water entering the plant. The present invention has no such requirement or step and can be deemed to be exclusively a data "feed forward" method, wherein the color and turbidity data of the untreated water is used exclusively to determine the optimum quantity of coagulant to be added.